1. Field of the Invention
The present invention relates to techniques for retrieving information about an object through nuclear magnetic resonance (NMR) signals. More specifically, this invention relates to techniques for increasing the amount of information which can be received from a single NMR signal.
2. Description of the Prior Art
Any information system has a finite dynamic range, which may be expressed in decibels, between maximum and minimum signal amplitudes which the system can receive, process, or transmit. For example, Tomlinson, U.S. Pat. No. 4,034,191, relates to a spectrometer, which transmits a series of pulsed signals into an object being analyzed. Tomlinson discusses a technique for reducing the peak power requirements of the transmitter to fit within a given dynamic range by modulating the pulse height or pulse width of the pulsed signals in accordance with a pseudorandom sequence, and more specifically discusses the addition of a pseudorandom phase shift component into each of the Fourier frequency components of a desired broadband excitation waveform used to modulate the pulses. This reduces the peak pulsed power requirement because the peak amplitudes of the frequency components occur out of phase with each other in the modulating waveform. Also, the phases of the simultaneously excited resonance spectral lines are scrambled.
In many NMR applications, rather than applying a series of pulses, during each of which a simultaneous NMR signal is received as in Tomlinson, a pulse and magnetic field gradient sequence is applied to obtain one or more NMR signals during a subsequent time interval. Although these NMR signals may have relatively high peak amplitudes, the average amplitude of each signal over the full detection period is quite low. In Fourier transform (FT) NMR imaging, for example, a transient response from a large number of spin systems is received as a function of time, and the maximum amplitude of this transient response is determined by the total number of spin systems which have been excited. Detailed information about the NMR spectrum is contained in low amplitude components of the spin response, however, and may be obtained by Fourier transformation of the detected time varying signal, provided that the dynamic range of the NMR imaging system is large enough to include both the peak amplitude of the transient response and the much lower amplitude components of the spin response. During most of its duration, the spin response will have a very low amplitude, particularly when the signal must be sampled for a long time period in order to obtain detailed frequency information while the transverse relaxation of the spin systems according to the time constant T.sub.2 * results in a significant loss of signal amplitude over the sampling period.
Therefore, it would be advantageous to have techniques making it possible to reduce the peak amplitude of the received NMR signal in NMR imaging. Reducing the peak amplitude would reduce the dynamic range of the information in a given NMR signal. Such an NMR signal could contain information filling the dynamic range of the NMR imaging system, thereby making it possible to receive more information using that system.